1. Field of the Invention
The present invention relates generally to a small-size two cycle combustion engine of a kind used as a drive source for a work machine or equipment such as, for example, a brush cutter. More specifically, the present invention relates to the two cycle combustion engine of a structure, in which a cylinder block is connectedly mounted on a crankcase that is divided into two crank casings in a direction axially of a crankshaft accommodated in the crankcase.
2. Description of the Prior Art
In the small-size combustion engine of a type utilizing the crankcase that is divided into two crank casings in a direction axially of the crankshaft, a connecting structure has been hitherto generally employed, in which fastening bolts inserted from above into corresponding insertion holes defined in a cylinder block flange at a lower portion of the cylinder block are threadingly engaged in associated internally threaded holes defined in the crankcase. Since in this known connecting structure, the fastening bolts are threaded successively into the respective internally threaded holes, from above in a direction axially of the cylinder, by the use of a bolt fastening tool, cooling fins integral with the cylinder block are required to be formed with a plurality of coaxially aligned series of cuts or throughholes (hereinafter referred to as series-aligned tool insertion holes), one series defined for each fastening bolt, for the passage of the bolt fastening tool therethrough. For this reason, the use of this known connecting structure has been found involving a problem associated with reduction in cooling performance of the cylinder block as the total surface area of the cooling fins is reduced in the presence of the series-aligned tool insertion holes in the cylinder block. Also, the presence of those tool insertion holes necessarily causes the cooling fins to represent rugged shapes particularly in a direction circumferentially of the cylinder block and also causes the thermal transmission to be uneven, eventually resulting in lowering of the cooling efficiency with which the cylinder block is cooled.
On the other hand, the Japanese Laid-open Patent Publication No. 2003-227405, published Aug. 15, 2003, for example, discloses another type of connecting structure, which does not require the use of the series-aligned tool insertion holes so that a relatively high efficiency of cooling the cylinder block can be maintained. According to this known connecting structure, a plurality of fastening bolts arranged circumferentially of that lower portion of the cylinder block are threaded to the crankcase in a fashion inclined relative to the cylinder axis to avoid interference with the cooling fins. With this known connecting structure, the cooling fins integral with the cylinder block do in no way interfere the passage of the bolt fastening tool and, hence, the fastening work performed with the bolt fastening tool and, therefore, the use of the series-aligned tool insertion holes is effectively eliminated. Accordingly, the total surface area of the cooling fins need not be sacrificed and, consequently, the efficiency of cooling of the cylinder block can be advantageously maintained at a high level.
However, the connecting structure disclosed in the above mentioned patent publication still has a problem. Specifically, while respective mating surfaces of the cylinder block and the crankcase lie perpendicular to the cylinder axis, bolt bearing surfaces for receiving the respective fastening bolts must be defined inclined relative to the plane of interface between the mating surfaces of the cylinder block and the crankcase. For this reason, not only is threading of the crankcase complicated and difficult to perform, but the fastening force, with which the crankcase and the cylinder block are bolted together, is lowered by a quantity corresponding to the extent of inclination of the bolt bearing surfaces.
In view of the foregoing, the present invention is intended to provide a two cycle combustion engine utilizing a connecting structure of a shape that can be formed inexpensively with a simple process while enabling the efficiency of cooling of the cylinder block to be maintained at a high level and, also, that can firmly connect the cylinder block with the crankcase with fastening forces exerted by the fastening bolts.